This invention relates to an opto-mechanical deflector for an imaging system.
Related prior art opto-mechanical deflectors are typically employed in a typographic setting device in order to project typographic characters along a deflection line into a planar face, or light-sensitive film plane. Such typographic setting devices including the prior art opt-mechanical deflector project a beam bundle or collimated light beam onto a rotatable or slewable scanning mirror, whence the beam is projected onto a flat image plane in such a manner that the relation between the deflection angle of the beam bundle at the rotatable mirror and the position of the beam bundle projected into a deflection line is linearized by the optical system between the rotatable mirror and the flat image plane. Such a system is disclosed in German Patent No. P 34 04 407.1, U.S. Pat. No. 4,629,283. In detail, this optical system includes, near the rotatable mirror, a plano-convex lens with an approximately planar face facing the rotatable mirror and a spherically-convex face facing the reflection plane or the flat image plane, and, following it, a field-flattening concave mirror. The spherically reflecting face of the concave mirror is directed to the flat image plane. Before the rotatable mirror, there may be arranged in the beam path an objective lens and a disperson lens which are to serve, inter alia, a pre-correction of the spherical aberration of the deflection system. Starting from an intermediate image plane into which an image point of a character can be projected, the beam bundle traverses through the dispersion lens, the objective lens and, preferably, through a spherically-convex face of a plane-convex lens and exits from its planar face as a bundle of collimated light. The later impinges on the rotatable reflecting face and, depending on the rotation position, is reflected into a plano-convex lens in order to finally be projected into the deflection line. The planar face of the plano-convex lens thereby reduces the deflection angle of the rotatable mirror required for the deflection of the position of the character projected in the deflection line. This reduction is substantially obtained by refraction at the planar face of the plano-convex lens. The beam bundle traversing through this lens is then, at its exit at its spherically-convex face, focused substantially into the position in the deflection line, i.e. further optical elements of the deflection system add correctingly to the focusing. The requirement that the plano-convex single lens be an aplanatic one, requres a relatively strong glass thickness of this single lens. While by means of this opto-mechanical deflector and the additional lenses, particularly the dispersion lens, in the entry beam path, a very good linearity between the deflection angle or the rotation position of the rotatable mirror and the projection position in the flat image plane is obtained if the deflection angle of the rotatable mirror is not too great, or the deflection line, at a given focal length of the plano-convex lens, is not too long. It is, however, in cases of particularly long deflection lines and high quality requirments desirable to still better correct the unavoidable aberration of the larger concave mirror provided near the image plane. It is therefore the aim of the present invention to further improve the correction of this aberration of the concave mirror in an opto-mechanical deflector system of the kind referred to in the beginning, particularly in case of long deflection lines.